intro optics 3

8/12/2019 Intro Optics 3

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Advancedo tics:retardationandinterferencefigures


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refraction in a mineral section.

ou e re rac on epen s on:

1. The orientation of the mineral section with respect to its

lattice.

2. The minerals birefringence.

Birefringence in anisotropic minerals is maximised when the

difference between the refractive indices is maximised.

To view this maximum we must examine a crystal section in a

specific orientation.


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Uniaxial Minerals

Minerals with two or three equal crystal axes that lie in a

p ane perpen cu ar to an ax s o a erent engt are a

UNIAXIAL.

This, single axis, parallel to the crystallographic c axis is also

called the minerals OPTIC AXIS.

Which crystallographic systems are we talking about?

When light is shone down the optic axis it behaves in the

same manner as expected when interacting with an

so rop c m nera .

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When light is shone in any other direction, what happens?

omet ng e se appens

too when the calcite

rhomb is rotated.

On rotation, one dot remains

,

rotates with the calcite.

The stationary light vibrates in the basal plane and no

matter the degree of rotation always travels the same

distance in the same time this is the Ordinar O wave.

The second beam, vibrating in the plane of the optic axis

.

(the)Extraordinary (E) (behaviour) wave.


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Uniaxial minerals are divided into those that are positive and negative.

When ordinary wave, which

travels an equal distance in all

directions has a reater velocit

the mineral is positive.

When ordinary wave, which

travels an equal distance in allrect ons, as a ower ve oc ty,

the mineral is negative.

The two light rays are indicative of two refractive indices in uniaxial crysals.

.

Vibration along the E wave is designated (Epsilon)


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Eye pointAll light passing from the source,

through the polariser, to the sampleEye piece travels parallel to the vertical.

s g , w en en er ng an

anisotropic crystal with the special

orientation optic axis parallel toec ve

lenses direction of light propagation

passes straight through and is not

Stage

Sample .

when viewed down the microscope

with the polarisers crossed.

Diaphragm

See earlier lecture.Lower Lens

Polarizer


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Eye point

Eye piece

the light is strongly converged and

only some of the light enters parallel

o e op c ax s.

Therefore, only some of the lightec ve

lensesenters the crystal and passes straight

through.

Stage

SampleAll light that enters the crystal

oblique to the optic axis is split into

Diaphragm

directions, and therefore is

susceptible to the normal processes

Lower Lens

Polarizerof retardation and birefringence.


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Identifying Uniaxial Crystals in

Convergent Polarized Light.Light is oblique to optic axis,

This is done by constructing an

interference figure (also known as

the o tical indicatrix .

.

The indicatrix is constructed so

plotted as radii that are parallel to

the vibration direction of the light.

Light parallel to optic axis enters mineral and is

not split in two, so passes straight through.

More realistically, you identify a mineral grain with no, to minimal variation in

birefringence (i.e. black in XPlrs), insert the sub-stage condenser, insert theBertrand lens, use highest magnification, and open the diaphragm.


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Off-centred uniaxial figures can also be used for identification

purposes. The limbs should always be horizontal and vertical,

even if the centre moves to edge of the field of view.

Optically +vewith tint plate

Optically -vewith tint plate


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Biaxial Minerals

Minerals with the lowest symmetry and three crystal axes of

unequa engt , an n most cases one nter-ax s ang e .

So called because they have two directions in which light

travels with zero birefringence (i.e. two OPTIC AXES).

Which crystallographic systems are we talking about?

g t enter ng ax a m nera s s ro en nto two rays afast one and a slow one.

However, both waves are extraordinary.

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Biaxial minerals have three unequal crystal axes.

Subsequently, these minerals have three

refractive indice values i.e. each value of n is.

mpor an : n e e crys a ograp c axes

the three optical directions are

always at right anglesto each other.


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We represent the three optical directions by

.

The birefrin ence is

maximum when a section iscut parallel to X and Z.C

Light travels parallel to Y.

B

A The angle between the two OA isknown as optic axial angle (2V).


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When 2V is acute around X

When 2V is acute around Z

the mineral is positive.


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If the mineral is positive, Z

is the acute bisectrix, and Xthe obtuse bisectrix.

negative minerals.

The optic axes always lie in

the XZ plane, which is

(OAP)


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What does the biaxial indicatrix look like?

Optic Axis

Plane

Melatopes (M) mark the

points of emergence of the

optic axis.

Only light entering parallel to an optic

axis passes without being split.


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Each line represents

an soc rome o

birefrengence

Each extinguished linein the optical image is

ca e an sogyre.

Looking down

the c axis.

If you have a crystal oriented so that you are looking straight

down the c axis ou can measure 2V usin sim le eometr .


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If ou have a cr stal oriented so that ou are lookin strai htdown one of the optic axis i.e. you only have one isogyre in the

field of view, it is still possible to estimate 2V.

2V = 0 2V = 90


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Crystal Optical Axes # of optic

System Class axes

Isometric Isotropic n Infinite

Tetragonal Uniaxial > (+)

< (-) 1Hexagonal

Orthorhombic

Biaxial closer 2

(+)

closer (-)Triclinic

IsotropicAnisotropic

Uniaxial positiveOpticaxis:perpendiculartothecircular

direction

Uniaxial negativeBiaxial positiveBiaxial negative

Acute

bisectrix

(2V):

angle

less

than

90

between2opticaxesinbiaxialminerals

Obtusebisectrix:bisectsthelargerangleina

ax a m nera

intro optics 3

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